1. Field of the Invention
The present invention relates to a midsole of a shoe sole, particularly to a cushioning structure thereof.
2. Description of the Related Art
A shoe sole is required to have cushioning performance.
In a conventional shoe sole, in general, a landing shock at the time of walking is absorbed by dissipating energy through compression deformation of a midsole or the like. However, a sufficient cushioning property can not be obtained merely by the absorption (dissipation) of the energy through compression deformation, since the amount of the absorption is generally small.
On the other hand, if the midsole is made thick in order to make the dissipation of the energy large, the lightweight property of the shoe sole is lost.
FIG. 15(a) is a perspective view of a cushioning part disclosed in Japanese Patent Laid-Open No. Hei8-38211.
This cushioning part 500 is made of gel, and is provided with notch portions 501 for allowing compression deformation at the time of compression deformation of the part 500. However, the notch portions 501 are not a significant factor in promoting shear deformation.
FIG. 15(b) is a cross-sectional vertical side view showing a cushioning structure disclosed in Japanese Patent Laid-Open No. Hei3-170102.
The cushioning structure shown in FIG. 15(b) is provided with a columnar part 510 made of gel, and a coil spring 511 fitted around the part 510 for storing repulsive “spring-back” energy at the time of kicking and going forward.
FIG. 15(c) is a perspective view showing a part of an orthopedic shoe sole disclosed in U.S. Pat. No. 4,217,907.
This part 520 is fixed to a heel of an outer sole. This part 520 includes a number of projecting ribs 521 arranged side by side in a circumferential direction. When receiving a repulsing force W from the ground, the projecting ribs 521 rotate part 520 in the direction of the arrow 522. The part 520 is for correcting and curing foot deformities by this rotation. Part 520 is made of a relatively hard material and is not designed to absorb shock.
FIG. 16(a) and FIG. 16(b) are a front view and a plan view respectively showing a projection 400 of a sole disclosed in Peterson (U.S. Pat. No. 5,782,014).
A midsole unit of Peterson is provided with the helical or screw-like projection 400. Groove 401 is provided around the projection 400 in a range α1 of rotation of 360 degrees or more, i.e., groove 401 completely circumscribes projection 400. Since projection 400 thus has a shape like a screw and if a compression load is applied vertically to projection 400, the projection 400 is vertically compression-deformed like a coil spring, i.e., there is only a minimal amount of shear deformation.
A cushioning structure disclosed in Japanese Patent Laid-Open No. 197503/2000 that includes a shearing transformation element at a rear foot portion of a midsole. The shearing transformation element is shear-deformed at the time of landing in such a manner that it falls forward. However, since the element is deformed in such a manner that it falls, it is difficult to apply this concept under the ball of the foot.